The purpose of this study is to follow a group of patients with AIDS who have had serious medical illnesses or persistently low CD4 counts, with evaluations of brain, nerve, and muscle function, done by experts who specialize in the care of AIDS patients. These patients qualify for this study because they are infected with HIV and have had a serious AIDS-related illness or persistently low CD4 counts for a period of 3 months. There is evidence that patients with this condition may have undiagnosed brain, nerve, and muscle disorders. Upon entering this study patients are being asked that in the event of their death, an autopsy can be performed. HIV-positive patients cannot provide organ donations, but can donate tissue for research to help scientists understand the causes of AIDS and how HIV causes brain, muscle, and nerve disorders.